shaft horsepower

Shaft horsepower (SHP) is the mechanical power delivered by a rotating shaft, measured in horsepower. It is what comes out of an engine or motor to do real work, as distinct from the theoretical power the engine produces internally. A turbine, pump, compressor, or gear transmission receives SHP from its input shaft and either uses it to drive a load or transforms it to deliver it elsewhere.

The term exists because engines and turbines produce indicated horsepower, the power developed inside the cylinders or rotor, but friction, windage, and mechanical losses consume some of that power. Brake horsepower is what remains when an engine is connected to a test dynamometer. Shaft horsepower is the next step, the power available at the shaft after gearboxes, bearings, and seals have taken their cut. In a marine diesel engine driving a propeller through a reduction gearbox, the SHP at the propeller shaft is significantly less than the engine's brake horsepower because gear losses are real and measurable.

Variants depend on what sits between the power source and the load. A direct-drive fan on a motor has high SHP relative to the motor's rated power because losses are minimal. A heavily geared industrial transmission can lose 3 to 5 percent of input power per mesh, so a 1000 brake horsepower motor feeding through a four-stage gearbox might deliver only 900 SHP at the output shaft. Fluid couplings, magnetic drives, and hydraulic transmissions introduce additional loss; their efficiency is published and must be factored in for system design.

SHP is critical in pump and compressor selection. A centrifugal pump rated for 200 gallons per minute at 100 feet of head requires a specific SHP input; undersizing the motor or overestimating motor efficiency will result in cavitation, surge, or stalling. In turbomachinery, SHP is the bridge between aerodynamic or hydraulic design and the electrical or mechanical supply. Marine classification societies and API standards require that SHP calculations account for all known losses and include a service factor, typically 1.15 for pumps, to ensure the system performs over its life.

The name reflects the historical use of horsepower as a unit tied to physical shaft rotation. Modern measurement uses torque and rotational speed, combined as Power (kW) = (Torque (N·m) × Speed (rpm)) / 9550, but horsepower persists in marine, power generation, and legacy industrial applications. SHP remains distinct from input electrical power consumed; a 1500 kW (roughly 2000 hp) electric motor at 90 percent efficiency delivers only 1350 kW (1800 hp) as SHP to the driven equipment.